在上一节，我们将静态实现AOP，但是对于一个大型项目，要想为每个类，每个方法都去实现AOP ,进行日志记录和权限验证似乎是不可能的。 即使可能对于成百上千个类维护，也是很难维护。所以今天的主题就是如标题所述。
（Real Proxy）真正代理和（Dynamic Proxy）动态代理
Real Proxy 类是个抽象类，定义在 System.Runtime.Remoting.Proxies，定义了代理的一些公共的基本的功能方法。
Dynamic Proxy 类必须继承Real Proxy，通过重写它的调用方法和添加新功能来达到目的。那么 Dynamic Proxy 定义如下：
class DynamicProxy<T> : RealProxy
{
private readonly T _decorated;
public DynamicProxy(T decorated)
: base(typeof(T))
{
_decorated = decorated;
}
private void Log(string msg, object arg = null)
{
Console.ForegroundColor = ConsoleColor.Red;
Console.WriteLine(msg, arg);
Console.ResetColor();
}
public override IMessage Invoke(IMessage msg)
{
var methodCall = msg as IMethodCallMessage;
var methodInfo = methodCall.MethodBase as MethodInfo;
Log("In Dynamic Proxy - Before executing '{0}'",
methodCall.MethodName);
try
{
var result = methodInfo.Invoke(_decorated, methodCall.InArgs);
Log("In Dynamic Proxy - After executing '{0}' ",
methodCall.MethodName);
return new ReturnMessage(result, null, 0,
methodCall.LogicalCallContext, methodCall);
}
catch (Exception e)
{
Log(string.Format(
"In Dynamic Proxy- Exception {0} executing '{1}'", e),
methodCall.MethodName);
return new ReturnMessage(e, methodCall);
}
}
}
为此，我们已经定义好了动态代理类，但是为了使用包装者Repository<T>类， 必须还得使用GetTransparentProxy（他返回一个 IRepository<Customer>实例）方法，每一个方实例的方法的调用将通过代理来调用，也就是说，你可以定义一个Factory 来创建 proxy和返回泛型Repository<T>实例。所以代码类似于定义如下：
public class RepositoryFactory
{
public static IRepository<T> Create<T>()
{
var repository = new Repository<T>();
var dynamicProxy = new DynamicProxy<IRepository<T>>(repository);
return dynamicProxy.GetTransparentProxy() as IRepository<T>;
}
}
接下来，编写控制台代码 大概如下：
static void Main(string[] args)
{
Console.WriteLine("*** Begin program - logging with dynamic proxy ");
// IRepository<Customer> customerRepository =
// new Repository<Customer>();
// IRepository<Customer> customerRepository =
// new LoggerRepository<Customer>(new Repository<Customer>());
IRepository<Customer> customerRepository =
RepositoryFactory.Create<Customer>();
var customer = new Customer
{
Id = 1,
Name = "Customer 1",
Address = "Address 1"
;
customerRepository.Add(customer);
customerRepository.Update(customer);
customerRepository.Delete(customer);
Console.WriteLine(" End program - logging with dynamic proxy ***");
Console.ReadLine();
}
运行结果，如下：
正如你所看到的,你已经创建一个动态代理,允许添加AOP代码,不需要重复。如果你想添加一个新的方面,现在你只需要创建一个新的类,继承Real Proxy并使用它来装饰第一个代理。如果你的老板要你,要求你添加授权代码,并且只有管理员才可以访问存储库,您可以创建一个新的代理,代码如下：
class AuthenticationProxy<T> : RealProxy
{
private readonly T _decorated;
public AuthenticationProxy(T decorated)
: base(typeof(T))
{
_decorated = decorated;
}
private void Log(string msg, object arg = null)
{
Console.ForegroundColor = ConsoleColor.Green;
Console.WriteLine(msg, arg);
Console.ResetColor();
}
public override IMessage Invoke(IMessage msg)
{
var methodCall = msg as IMethodCallMessage;
var methodInfo = methodCall.MethodBase as MethodInfo;
if (Thread.CurrentPrincipal.IsInRole("ADMIN"))
{
try
{
Log("User authenticated - You can execute '{0}' ",
methodCall.MethodName);
var result = methodInfo.Invoke(_decorated, methodCall.InArgs);
return new ReturnMessage(result, null, 0,
methodCall.LogicalCallContext, methodCall);
}
catch (Exception e)
{
Log(string.Format(
"User authenticated - Exception {0} executing '{1}'", e),
methodCall.MethodName);
return new ReturnMessage(e, methodCall);
}
}
Log("User not authenticated - You can't execute '{0}' ",
methodCall.MethodName);
return new ReturnMessage(null, null, 0,
methodCall.LogicalCallContext, methodCall);
}
}
所以repository工厂必须更改实现两个代理，代码如下：
public class RepositoryFactory
{
public static IRepository<TCreate<T>()
{
var repository = new Repository<T>();
var decoratedRepository =
(IRepository<T>)new DynamicProxy<IRepository<T>>(
repository).GetTransparentProxy();
// Create a dynamic proxy for the class already decorated
decoratedRepository =
(IRepository<T>)new AuthenticationProxy<IRepository<T>>(
decoratedRepository).GetTransparentProxy();
return decoratedRepository;
}
}
控制台程序，需要更改代码，如下所示：
static void Main(string[] args)
{
Console.WriteLine(
"*** Begin program - logging and authentication ");
Console.WriteLine(" Running as admin");
Thread.CurrentPrincipal =
new GenericPrincipal(new GenericIdentity("Administrator"),
new[] { "ADMIN" });
IRepository<Customer> customerRepository =
RepositoryFactory.Create<Customer>();
var customer = new Customer
{
Id = 1,
Name = "Customer 1",
Address = "Address 1"
};
customerRepository.Add(customer);
customerRepository.Update(customer);
customerRepository.Delete(customer);
Console.WriteLine(" Running as user");
Thread.CurrentPrincipal =
new GenericPrincipal(new GenericIdentity("NormalUser"),
new string[] { });
customerRepository.Add(customer);
customerRepository.Update(customer);
customerRepository.Delete(customer);
Console.WriteLine(
" End program - logging and authentication ***");
Console.ReadLine();
}
运行结果如下
到此为止就 全部实现动态代理实现AOP.
下一节，我们讲继续讲Aop之方法过滤。
AOP参考
本文翻译自 ：https://msdn.microsoft.com/en-us/magazine/dn574804.aspx（面向切面编程）